1. Field of the invention
This invention is related to see-through packaging structures, for liquid or semi-liquid fruit or vegetable products. The structures have a polyamide food contact-layer, which extends the color-life of the contents. The polyamide layer removes many brown polyphenolic degradation products of various compounds present in fruits and vegetables which otherwise would lead to browning of the contents during storage. More particularly, the invention comprises any see-through packaging structure through which the liquid contents and their color can be clearly seen such as a film pouch or plastic bottle, which is formed from a transparent polymeric film or sheet having a polyamide surface-layer, the polyamide layer becoming an inner contact-layer of the packaging structure.
2. Discussion of Related Art
Fruit and vegetable juices discolor on aging for several reasons. One of these is formation of certain brownish polyphenolic materials via a non-enzymatic reaction. These polyphenolic compounds include certain `degradation` products of anthocyanins, and of flavonoids, proanthocyanins and tannins, which themselves are polyphenolic compounds present in foods. The `degradation` products may in many cases form from polycondensation reactions. Anthocyanins and related compounds, for instance, may be the desirable color giving substances in some foods, but with time can form undesirable brown polyphenolics. Proanthocyanidines are colorless, but their degradation products are colored. Browning is an especial problem in light-colored fruit and vegetable products such as strawberry, apricot, and orange juices, jams or jellies.
Polyamides have long been used as absorbents and filters ("clarifying agents") in the processing of beer, wine, fruit juice, and vegetable juices to remove certain undesirable polyphenolic compounds present in the fresh food product. For example, Japanese patent J82053134-B (assigned to Asahi Chemical and Ink) discloses an absorptive sheet containing polyamides and inorganic adsorbent for use in processing, but not in the storage or packaging of vegetable juices.
The mechanism by which polyamides remove these undesirable polyphenolic compounds is believed to be adsorption due to hydrogen bonding between phenolic hydroxyl groups and amide bonds in the protein `polyamides`. Clarifying agents are used after the juice is produced (and fermented, in the case of beer and wine) but before it is packaged (ref.: Food Chemistry, Owen R. Fennema, ed., 1985, p 661-663).
Polyamides have also been used as the food contact-layer in food packaging to prevent `scalping` (adsorption or absorption of essences which are taste components) by the package, but these are typically used in opaque or near opaque structures which also prevent or reduce light ingress, and where the color of the food is not readily seen by the consumer. Opaque packaging is frequently deliberately used because light can induce the formation of certain other types of degradation reactions. As an example of this type of packaging, unpasteurized, not-from-concentrate orange juice has been packaged in coextruded structures of nylon and HDPE as disclosed in `Packaging Strategies` (11 (14), 2 (1993)). Nylon is used as the contact-layer because it does not scalp flavor. The HDPE is not transparent and the polyamide may not be depending on its type and thickness in the package. In this instance, the polyamide acts as a barrier to prevent adsorption or absorption of desirable compounds, (the flavor components), by the other layer or layers. By contrast, the purpose of the present invention is just the opposite; specifically to adsorb or absorb undesirable compounds, namely the brown degradation products.
Polyamides are also often used as an oxygen barrier-layer in packaging structures. However, because the oxygen barrier properties of polyamides are moisture sensitive, they are incorporated as an inner-layer of a multilayer structure when packaging liquid or semi-liquid foods with high moisture content. For example, Japanese Patent Publication No. 75-010196B describes a specific polyamide containing m-xylylenediamine which can be used in an oriented structure for packaging water-containing substances to prevent discoloration and spoilage, but the polyamide does not contact the product. In these structures, since the polyamide does not contact the product, it cannot remove polyphenolic compounds but it can act as an oxygen barrier.
Since some of the causes of browning are enzymatic, packaging these products in a structure with excellent oxygen barrier or gas flushing to remove oxygen will minimize other color changes. For example, a study of orange juice aged in various containers and analyzed over time for changes in browning, based on the level of hydroxymethyl furfural (ascorbic acid degradation product), showed that removal of oxygen and use of packaging with high oxygen barrier improved quality over time (J. Japan. Soc. Food Sci Tech 30 (8), 467 (1983)).
Similarly, studies on manufacture and packaging of candied fruits show that exclusion of oxygen (by using film with low oxygen permeability and/or vacuum packaging) reduces mold growth and browning of fruits (Taiwan Food Science 13 (3/4), 117 (1986)).
A study of multilayer films evaluated as packaging materials for hot-filled fruit products, comparing EVOH, PVDC, or nylon as the food contact-layer and applesauce as the fruit product showed that nylon performed worst (J Food Sci. 57 (3), 671 (1992)). Since apple sauce browns mainly by enzymatic (oxidative) browning, this result may be related to the effectiveness of the film as an oxygen barrier.
There is a need and desire for see-through packaging structures which are colorless, allowing the contents and color of liquid and semi-liquid food materials contained therein to be readily seen, yet which also strongly reduce non-enzymatic discoloration of those foods due to certain non-enzymatically formed polyphenolic degradation compounds.